1. Field of the invention
The present invention relates to a process for manufacturing an elongated article made of sintered ceramic having superconductivity.
Particularly, it relates to a process for manufacturing a superconducting wire made of sintered ceramic of compound oxide which is applicable for producing a superconducting coils or the like.
More particularly, the present invention relates to a process for manufacturing a superconducting wire made of sintered ceramic of compound oxide having higher critical current density and higher critical transition temperature of superconductivity.
2. Description of the related art
The superconductivity is a phenomenon in which the electrical resistance become zero and hence can be utilized to realize power cables and a variety of devices and apparatus which are requested to reduce consumption of electrical energy and several ideas of its applications which utilize the phenomenon of superconductivity have been proposed.
In fact, the superconductivity are applicable in a variety of industrial fields, for example in the field of electrical power supply such as fusion power, MHD power generation, power transmission, or electric power reservation; in the field of transportation such as magnetic levitation trains, magnetically propelling ships; in the medical field such as high-energy beam radiation unit; in the field of science such as NMR or high-energy physics; or in the field of sensors or detectors for sensing very weak magnetic field, microwave, radiant ray or the like as well as in the field of electronics such as Josephson Junction devices and high-speed computers with reduced energy consumption.
However, their actual usage have been restricted because the phenomenon of superconductivity can be observed only at very low cryogenic temperatures. Among known superconducting materials, a group of materials having so-called A-15 structure show rather higher Tc (critical temperature of superconductivity) than others, but even the top record of Tc in the case of Nb.sub.3 Ge which showed the highest Tc could not exceed 23.2 K at most.
This means that liquidized helium (boiling point of 4.2 K) is only one cryogen that can realize such very low temperature of Tc. However, helium is not only a limited costly resource but also require a large-scaled system for liquefaction. Therefore, it had been desired to find another superconducting materials having much higher Tc. But no material which exceeded the abovementioned Tc had been found for all studies for the past ten years.
It is known that certain ceramics material of compound oxides exhibit the property of superconductivity. For example, U.S. Pat. No. 3,932,315 discloses Ba--Pb--Bi-type compound oxide which shows superconductivity. This type superconductor, however, possess a rather low transition temperature of lower than 13 K and hence usage of liquidized helium (boiling point of 4.2 K) as cryogen is indispensable to realize superconductivity.
Possibility of existence of a new type of superconducting materials having much higher Tc was revealed by Bednorz and Muller who discovered a new oxide type superconductor in 1986 (Z. Phys. B64 (1986) p189)
This new oxide type superconducting material is [La, Ba].sub.2 CuO.sub.4 or [La, Sr].sub.2 CuO.sub.4 which are so-called the K.sub.2 NiF.sub.4 -type oxide having such a crystal structure that is similar to Perovskite-type superconducting oxides which were known in the past (for example, BaPb.sub.1-x BixO.sub.3 disclosed in U.S. Pat. No. 3,932,315). The K.sub.2 NiF.sub.4 -type oxides show such higher Tc as about 30 K which is extremely higher than that of known superconducting materials.
As the compound oxide type superconductors consisting of oxides of elements of IIa and IIIa groups in the Periodic Table, it can be mentioned those of, so to say, quasi-Perovskite structure which can be considered to have such a crystal structure that is similar to Perovskite-type oxides and includes an orthorhombically distorted perovskite or a distorted oxygen-deficient perovskite such as Ba.sub.2 YCu.sub.3 O.sub.7-.delta. in addition to the abovementioned K.sub.2 NiF.sub.4 -type oxide such as [La, Ba].sub.2 CuO.sub.4 or [La, Sr].sub.2 CuO.sub.4. Since these superconducting materials show very high Tc of 30 to 90 K, it becomes possible to use liquidized hydrogen (b.p.=20.4 K) or liquidized neon (b.p.=27.3 K) as a cryogen for realizing the superconductivity in practice. Particularly, hydrogen is an inexhaustable resource except for danger of explosion.
However, the above mentioned new type superconducting materials which was just born have been studied and developed only in a form of sintered bodies as a bulk produced from powders but have not been tried to be shaped into a wire form. The reason is that the new type superconductors are ceramic materials of compound oxide which possess no superior plasticity or processablility in comparison with well-known metal type superconducting materials such as Ni--Ti alloy, and therefore they can not or are difficult to be shaped or deformed into an elongated article such as a wire by conventional technique such as wire-drawing technique in which superconducting metal is drawn directly or in embedded condition in copper to a wire form.
It is proposed in Japanese patent laid-open No. 61-131,307 a method for manufacturing a superconducting wire from a metal type superconducting material which is apt to be oxidized and very fragile such as PbMo.sub.0.35 S.sub.8, comprising charging the material powder in a metal shell, extruding the metal shell filled with the material powder at higher than 1,000.degree. C., and then drawing the extruded composite. This metal working technique, however, can not apply directly to ceramic material consisting of compound oxide, because the compound oxide type superconducting materials can not exhibit the superconductivity if not the specified or predetermined crystal structure is realized. In other words, a superconducting wire having higher critical temperature and higher critical current density and which is useable in actual applications can not be obtained outside predetermined optimum conditions. In particular, if not the shell is selected from proper materials, the resulting compound oxide will be reduced due to chemical reaction with the metal of the shell, resulting in poor or inferior properties of superconductivity.
In the field of ceramic molding, it has been the general practice for manufacturing an elongated article such as wires or rods to add an organic binder to the material powder of ceramic in order to facilitate shaping or molding of the powder material. Thus, a mixture of the powder material and the organic binder is shaped into a rod by means of an extruder or a press machine and then the shaped rod is passed directly or through a trimming or cutting stage to an intermediate sintering stage to remove the organic binder before it is fed to the final sintering stage.
The combination of the abovementioned press-molding and trimming or cutting operations loose much material of expensive ceramics, so that not only economy of material is low but also a dimensional ratio of longitudinal direction to cross sectional direction of the rod can not be increased. Therefore, this process can not be used in practice.
The extrusion technique is much better than the press-molding technique in the economy of material and productivity, but requires great quantities of organic binder added to the powder material. This organic binder is difficult to be removed completely during the intermediate sintering stage and hence remain in the finally sintered article, resulting in a cause of defects of the product which will lower the strength and the resistance to flexion. Therefore, it is difficult to manufacture a fine rod of ceramics having higher dimensional ratios of longitudinal direction to cross sectional direction according to the extrusion technique.
In order to realize a reliable and practical superconducting structure, it is indispensable that the structure possesses enough strength and tenacity which is sufficient to endure bending force during usage and also has a finer cross sectional dimension as possible in such manner that it can transmit currency at higher critical current density and at higher critical temperature.
Therefore, an object of the present invention is to provide a process for manufacturing a superconducting wire of sintered ceramic having an enough length to be used in practical applications, namely having a higher dimensional ratio of longitudinal direction to cross sectional direction, without using organic binder which is causative of lowering the strength and tenacity of the product.
Another object of the present invention is to provide a process for manufacturing a fine superconducting wire of compound oxide type sintered ceramic having higher resistance to breakage, even if the the diameter of the wire is reduced greatly, in other words, under higher dimensional reduction ratio in cross section.
Still another object of the present invention is to provide a process for manufacturing a fine superconducting wire of compound oxide type sintered ceramic having higher critical current density and higher critical temperature.